Many semiconductor processes such as chemical vapor deposition (CVD) employ vaporized processing liquids. These vaporized processing liquids are generated and supplied to a processing chamber via a processing liquid delivery system comprising an interconnection of pipes, valves, flow regulators and vaporizing mechanisms. Typically a separate vaporizing mechanism is provided for vaporizing each processing liquid, and is coupled to a source of processing liquid and a source of carrier gas. Although a number of vaporizing mechanisms exist (e.g., bubblers, injection valves, etc.), most conventional processing liquid delivery systems employ a plurality of injection valves for vaporizing processing liquids to be delivered to a processing chamber.
A typical injection valve comprises a processing liquid inlet for receiving a pressurized processing liquid, a carrier gas inlet for receiving a pressurized inert carrier gas, and an outlet for delivering a vaporized processing liquid/carrier gas mixture. The injection valve is heated such that when the processing liquid is injected into the carrier gas, the heat and the pressure difference between the two sides of the injection valve cause the processing liquid to vaporize.
Over time injection valves may clog or fail (e.g., due to deposit formation within the injection valve from the interaction of processing liquid with other processing chemicals or with the injection valve itself) and must be replaced. However, the process of injection valve replacement is complicated when the processing liquid vaporized by the injection valve reacts deleteriously with air (e.g., with moisture, oxygen, etc.) to form by-products (e.g., solid films such as oxides) that can damage the processing liquid delivery system or the processing chamber, contaminate subsequently processed semiconductor wafers or harm humans or the environment (e.g., are toxic).
To prevent deleterious processing liquid formation during injection valve replacement, if possible, processing liquid is purged from all processing liquid delivery lines that will be exposed to atmosphere when the clogged injection valve is removed. However, as described with reference to FIG. 1, within conventional processing liquid delivery systems the purging process is difficult, particularly when processing liquids with strong adhesive properties such as metal-organics (e.g., tetrakis(dimethylamino)titanium (TDMAT)) must be purged from processing liquid delivery lines.
FIG. 1 is schematic view of a conventional processing liquid delivery system 11 ("conventional system 11") for delivering vaporized processing liquid to a processing chamber 12. The conventional system 11 comprises a source of processing liquid 13 operatively coupled (i.e., coupled either directly or indirectly so as to operate) to an injection valve 15 via a processing liquid delivery line 17. Note that the processing liquid delivery line 17 is shown broken to indicate that the source of processing liquid 13 may be a substantial distance (e.g., about 10-15 feet) from the injection valve 15.
Disposed along and forming a part of the processing liquid delivery line 17 are a first isolation valve 19, a second isolation valve 21, a liquid flow meter 23 and a third isolation valve 25. The first isolation valve 19 is positioned near the source of processing liquid 13, the third isolation valve 25 is positioned near the injection valve 15, the liquid flow meter 23 is positioned near the third isolation valve 25, and the second isolation valve 21 is positioned near the liquid flow meter 23, as shown. A large number of other isolation valves typically are present along the processing liquid delivery line 17 but are omitted for clarity.
The conventional system 11 also comprises a source of purging gas 27 (e.g., nitrogen, argon, or some other gas which does not react with the processing liquid) operatively coupled to the processing liquid delivery line 17 via a purging gas line 29, and a pump 31 (e.g., a mechanical pump) operatively coupled to the processing liquid delivery line 17 via a pump line 33. Disposed along and forming a part of the purging gas line 29 is a purge valve 35, and disposed along and forming a part of the pump line 33 is a pump valve 37.
During normal operation of the conventional system 11, the first isolation valve 19, the second isolation valve 21 and the third isolation valve 25 are open to allow processing liquid to flow from the source of processing liquid 13 to the injection valve 15 at a rate controlled by the liquid flow meter 23. The purge valve 35 and the pump valve 37 are closed to prevent processing liquid from being purged by the source of purging gas 27 and from being pumped by the pump 31.
If the injection valve 15 subsequently becomes clogged and must be replaced, the injection valve 15 is isolated from the source of processing liquid 13 by closing the first isolation valve 19. Assuming the processing liquid is a metal-organic substance such as TDMAT, the injection valve 15 cannot be directly disconnected from the conventional system 11 without posing a substantial health risk to the technician removing the injection valve 15 and without posing a substantial damage risk to the conventional system 11. TDMAT, for instance, reacts with moisture in the air to form by-products that are harmful to humans (e.g. amines) and solid films (e.g., oxides) that will contaminate the entire conventional system 11. Processing liquid, therefore, must be purged from the processing liquid delivery line 17 prior to removing the injection valve 15.
To purge processing liquid from the processing liquid delivery line 17, while the first isolation valve 19 is closed and the second isolation valve 21 and the third isolation valve 25 are open, the purge valve 35 and the pump valve 37 are opened. Purging gas thereby flows from the source of purging gas 27, through the purging gas line 29, through the processing liquid delivery line 17 and through the pump line 33 to the pump 31. The purging gas dislodges processing liquid particles from the surfaces of the processing liquid delivery line 17, and the dislodged particles are pumped from the processing liquid delivery line 17 via the pump 31. Pump/purge cycles (wherein the purge valve 35 is closed for a time period while the pump 31 continues to pump processing liquid and purging gas from he processing liquid delivery line 17, followed by a time period wherein the purge valve 35 is opened so as to introduce more purging gas to the processing liquid delivery line 17) may be performed to aid in processing liquid removal from the processing liquid delivery line 17.
For processing liquids having strong adhesive properties (e.g., metal-organics), the pump/purge process described above does not effectively removing processing liquid from the processing liquid delivery line 17 to a level sufficient to prevent deleterious by-product formation when the injection valve 15 is disconnected from the conventional system 11. This is particularly true for TDMAT.
One approach to improving the purging effectiveness of the conventional system 11 is to employ thermal methods which heat the relevant processing liquid path to desorb processing liquid therefrom. Thermal methods, however, can damage rubber parts (e.g., valve seats), and can lead to decomposition of the processing liquid, generating particles and the problems associated therewith. Rubber parts, with added expense, can be designed to withstand thermal desorption temperatures. Decomposition, on the other hand, is unavoidable because processing liquid desorption and processing liquid decomposition may occur in the same temperature range. Many semiconductor processing liquids such as (TDMAT, dimethyl aluminum hydride (DMAH), (Trimethylvinylsilyl)hexafluoroacetylacetonato Copper 1 (CupraSelect.RTM.), etc.) deposit metal constituents as they decompose. The deposited metal may clog the processing liquid delivery line or clog downstream valves, and thus may further increase downtime costs. Even if clogging does not result, metals deposited in the processing liquid delivery line can flake therefrom, contaminating the processing chamber and potentially destroying any wafers being processed therein.
Accordingly, a need exists for a processing liquid purging method and apparatus that more effectively purges a processing liquid from a processing liquid delivery system without causing processing liquid decomposition and its attendant problems.